Digital cameras with a solid-state image sensor, in particular for example CMOS image sensors comprising a plurality of image pixels, located at a distance of a lens, are known in the art.
One of the problems with such cameras or image sensors is the problem of “shading” of the output signal provided by the pixels. Shading is the effect of a non-constant low spatial frequency output signal of the image sensor, even under uniform lighting conditions. This can be due to several reasons, for example:
1) Natural vignetting of the lens: all lenses have an intensity fall-off towards the corners of the sensor (away from the optical axis). This decrease in intensity of the incoming signal can be empirically described (cos4-law), and is depending on the focal length f and the F number of the lens. An example of the natural lens vignetting is shown in FIG. 1A. In the embodiment illustrated, the intensity decrease goes from 100% at the centre of the lens to 52% at the corners, but of course another value is also possible. An example of a fall-off curve is illustrated underneath the represented lens in FIG. 1A, but this is not limiting the present invention in any way. Other fall-off curves do occur. The lens vignetting cannot be avoided, even not in the most expensive lenses for professional applications.
2) Sensor vignetting: light rays fall nicely perpendicular on the sensor when they are parallel to the optical axis. But that is not the case in the corners of the sensor. Here the incoming rays come to the sensor with a deviating angle (known as the “chief ray angle”) to the normal. It is well known that the sensitivity of the sensor is the highest for perpendicularly incoming light. Thus pixels that receive the incoming information under an angle deviating from the normal to the sensor provide a reduced output signal. It should be noted that this effect is depending on the focal length f of the lens, the F number of the lens, but also on the wavelength of the incoming radiation (due to the dispersive characteristics of the materials). A simple example of sensor vignetting is shown in FIG. 1B. In this figure the sensor vignetting shown is illustrated without taking into account the dependency on the wavelength.
3) Filter vignetting: most colour cameras use several optical filters in front of the image sensor (e.g. IR-filter, optical low-pass filter, etc). The transmission characteristics of these filters depend also on the angle of the incoming rays, as well as on the wavelength of the incoming light. In other words, the external filters add to the shading as well. An example of the vignetting caused by an IR-filter is shown in FIG. 1C. As can be seen from the figure, the transmission in the corners (e.g. 90%) is reduced compared to the centre transmission (e.g. 100%). But moreover, the attenuation is different for the various colours (e.g. 90% for red, 97% for green, 99% for blue). The red spectrum suffers most from this type of vignetting.
4) It can be seen from the above that there are several contributions to the shading. Ideally all shading components are nicely circular-symmetric w.r.t. the optical axis, but unfortunately that is not always the case. The problem becomes even more complicated when the shading of the three colours components has a different shape, or when their optical axes do not coincide. FIG. 1D shows an example of non-circular-symmetric colour shading (although not very well visible on a black and white picture). The top left of the image has too much red output, while the bottom right has too much blue output. This figure shows that the shading components (R, G, B) may change for every pixel location of the image sensor.
Methods for correcting shading effects, once it is known how much shading occurs, are known in the art. US2002025164A1 describes an image sensor 100 (illustrated in FIG. 2) for providing in-situ image correction values. The image sensor 100 has (using the terminology of the cited document) a “light receiving region” 110, consisting of an “effective pixel part” 110A, an “available pixel part” 110B, and an “optical black region” 110C, the latter being covered by a light shielding film 114. The effective pixel part 110A contains only image pixels 120 (for capturing image data), and thus does not contain non-image pixels (for any other purpose). The “available pixel part” 110B contains non-image pixels, in particular blocks of 3×3 shading detection pixels 130. The shading correction pixels 130 are organized in blocks A, B, C, D, E, F, G of 3×3 pixels, as illustrated in more detail in FIG. 3 and FIG. 4. As can be seen, these blocks are located outside of the “effective pixel part” 110A defined by the image pixels 120. They can thus only measure shading information at locations near the edge and corner of the “effective pixel part” 110A (i.e. the rectangular region defined by the “image pixels” 120). It is also clear from FIG. 2 that the “light receiving region” 110 comprises the sum of the areas of the “effective pixel part” 110A and the “available pixel part” 110B, and therefore is larger (i.e. occupies more space) than the “effective pixel part” 110A. This extra space (substrate area such as e.g. silicon area) does not yield extra image data, thus does not yield a higher resolution picture, and is therefore not efficiently used.
Two kinds of shading detection pixels are disclosed in US2002025164A1. A first kind of shading detection pixels is illustrated in FIG. 3, where an array of 3×3 shading pixels 130 is covered by a metal light shield 132. The light shield 132 holds small openings 131 through which the light can hit the pixels 130 underneath. The openings 131 in the metal shield 132 have an offset from the pixel centre (except for the pixel in the middle of the 3×3 array). A second kind of shading pixels is illustrated in FIG. 4, where each pixel 130 comprises a micro-lens 133, each micro-lens 133 having a predefined offset from the pixel centre. The amount of shading can be derived from the nine values provided by the shading pixels 130 when radiation falls onto these. A disadvantage of this image sensor 100 of FIG. 2 is that its light sensitive area is not efficiently used.